In a conventional overhead view image generator, an overhead view image from directly above a vehicle is generated using an image transformation principle in which an image captured by an on-board camera is projected on a projection plane in parallel with a road surface, in other words, using a projective transformation in which a virtual viewpoint is positioned vertically above the vehicle. By displaying the overhead view image on a monitor, a driver is allowed to grasp a road surface state around the vehicle in a bird's-eye way. However, in the overhead view image obtained through such a projective transformation, a three-dimensional object standing on the road surface is elongated in an image capturing direction of the camera, and thus it becomes difficult for the driver to grasp a shape of the actual three-dimensional object from the overhead view image. In addition, when the overhead view image that covers the entire surroundings of the vehicle is generated from the captured images of a plurality of the on-board cameras, image composition of common regions in the adjacent captured images, i.e. overlap regions, has been generally performed. In the case of such an overhead view image, the image capturing directions of the respective original captured images of the overlap regions differ to a large extent, and the three-dimensional object present in the overlap region is elongated in two directions. As a result, the three-dimensional object appears as two three-dimensional object images in deformed shapes, and it becomes difficult to recognize the three-dimensional object, especially a standing position thereof, making a sense of a distance between the vehicle and the three-dimensional object ambiguous.
In order to solve such a problem, for example, Japanese Unexamined Patent Application Publication No. 2010-251939 (paragraphs [0004]-[0056], FIGS. 7 and 10) proposed a vehicle surroundings image display system including: a camera for capturing an image of vehicle surroundings; an obstacle detection means for detecting an obstacle in the vehicle surroundings; a memory section for storing in advance a substitute image corresponding to the obstacle; and an image processing section for generating the overhead view image of the vehicle surroundings from the virtual viewpoint based on the image captured by the camera, wherein, when the obstacle detection means detects the obstacle (three-dimensional object), the image processing section identifies the obstacle, selects a substitute image corresponding to the identified obstacle, reads the selected substitute image from the memory section, adjusts a direction and an inclination of the selected substitute image in accordance with the virtual viewpoint, and superimposes the adjusted image on the overhead view image. In this system, the image processing section transforms the image captured by an imaging means into the overhead view image from above the vehicle or from a side of the vehicle, and based on information from the obstacle detection means, the image processing section selects the substitute image from the memory section which is consistent with a size and movement of the obstacle present in the vehicle surroundings, and superimposes the selected image on the same overhead view image.
However, in a case where the substitute image in a shape of a human is displayed instead of an obstacle image showing the actual obstacle, if the shapes are different to a large degree between the substitute image and the actual obstacle, it becomes difficult for the driver to identify the substitute image as the obstacle actually confirmed with the naked eye, hindering a rapid obstacle recognition.
In view of the above, there has been a demand for an image generator for generating the monitor display image that facilitates easy recognition of the three-dimensional object in the overhead view image, especially a standing position of the three-dimensional object.